Overdrive on a TH 400 ??? That is new to me. I will have to research the hell out of that one.
I think the trans you have is what they called a "switch pitch"
I don't know much about them only that they are pre- 1967
I quickly copied this from The Hotrodders site.
((Electrical Connections))
Two different kickdown switches have been used. A 1 prong and the 2 prong switch.
The second prong on that connector is for the high-gear vacuum switch that directs vacuum to your distributor advance unit in high-gear only (an emissions item that doesn't affect performance). It only allows the solenoid to open in high-gear; I don't know if it is normally closed and only opens when a switch in the transmission (which, I suspect, is absent from the transmission he installed in there; it's specific to the early '70's trannys, and part of the valve body, so it's more than just the proper connector).
The second terminal you mentioned, is for the electric solenoid on top of the thermostatic vacuum control valve on the front of the intake manifold. There was a big ol electric switch up there, that, when a pressure switch inside the tranny saw pressure for third gear, it would ground the solenoid, and pull the valve up to port vacuum to the distributor. That is the reason for the "second" terminal. Not all trannys had this, and it's no big deal if yours don't, unless you are restoring to factory original status. If you routed your distributor vacuum to a ported source, you don't have any use for that switch.
To check the kickdown solenoid, you can do one of two things, you can either hook up a 12 volt source to it and listen for the "click" or you can turn the ignition to the "ON" position, and have someone floor it to activate passing gear. You should hear the click. Also, make DANG sure you know which wire goes where. It may be a case of just swapping the wiring in the terminal boot.
((Kickdown))
The electrical connection on the TH-400 is for the downshift solenoid. Unlike other GM transmissions, the TH-400 does not use a downshift/T.V. cable. The kickdown switch is mounted at the accelerator pedal under the dash. It sends 12 volts to a connector on the side of a TH-400 case during full throttle conditions. This then energizes a solenoid inside the trans providing downshifts for passing as well as higher rpm shift points for max acceleration.
